limit the invasiveness of the implanting embryo so it stays in the endometrium and does not reach the myometrium 811 •Fig. 44.26 Diagramoftheorganizationofglandsandbloodflowwithintheuterineendometrium.(FromStrausIII.In:YenSSCetal[eds].Reproductive Endocrinology. 4thed.Philadelphia:Saunders;1999.)Radial branch Spiral artery Uterine gland ENDOMETRIUM MYOMETRIUM Uterine artery Functional Zone Basal Zone Venous lakes Arcuate
CHAPTER 44 The Male and Female Reproductive Systems 4.
provide a maternal side of the mature placental architecture, including the basal plate to which the fetal side attaches, and large intervillous spaces that become filled with maternal blood after the first trimester 5.
grow and expand with the growing fetus so it develops within an aqueous nonadhesive environment 6.
provide strong muscular contractions to expel the fetus and placenta at term To understand the function of the uterus and uterine changes during nonfertile menstrual cycles, the fine structure of the endometrium and the relationship of uterine blood supply to the endometrium will be reviewed (Fig. 44.26). The luminal surface of the endometrium is covered with a simple cuboidal/columnar epithelium. The epithelium is •Fig. 44.27 Themenstrualcycleoftheuterineendometrium.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.) continuous with mucosal glands (called uterine glands) that extend deep into the endometrium. The mucosa is vascularized by spiral arteries, which are branches of the uterine artery that run through the myometrium. The terminal arterioles of the spiral arteries project just beneath the surface epithelium. These arterioles give rise to a subepithelial plexus of capillaries and venules that have ballooned thin-walled segments called venous lakes or lacunae. The lamina propria itself is densely cellular. The stromal cells of the lamina propria play important roles during both pregnancy and menstruation.
About two-thirds of the luminal side of the endometrium is lost during menstruation and is called the functional zone (also called the stratum functionalis) (see
Fig.
44.26 ). The basal third of the endometrium that remains after menstruation is called the basal zone (also called the stratum basale). The basal zone is fed by straight arteries that are separate from the spiral arteries, and it contains all the cell types of the endometrium (i.e., epithelial cells from the remaining tips of glands, stromal cells, and endothelial cells).
Hormonal Regulation of the Uterine Endometrium During the Menstrual Cycle
Monthly oscillations in ovarian steroids induce the uterine endometrium to enter different stages. At the time of selection of the dominant follicle and its elevating production of estradiol, the uterine endometrium is just ending menstruation. The stratum functionalis has been shed and only the stratum basale remains (
Fig. 44.27).
The rising levels of estrogen during the mid to late follicular phase of the ovary induce the proliferative phase of the uterine endometrium. Estrogen induces all cell types in the stratum basale to grow and divide. In fact the definition of an “estrogenic” compound has historically been one that is “uterotropic.” Estrogen increases cell proliferation directly through its cognate receptors (ER-α and ER-β), which regulate gene expression (
Fig. 44.28 ). Estrogen also controls uterine growth indirectly through local production of growth factors. In addition, estrogen induces expression of progesterone receptors, thereby “priming” the uterine endometrium so it can respond to progesterone during the luteal phase of the ovary.